Method of treating paper prepared from partially acetylated cellulose fibers



United States Patent Ofiice 2,992,156 Patented July 11, 1961 2,992,156 METHOD OF TREATING PAPER PREPARED FROM PARTIALLY ACETYLATED CELLULOSE FIBERS Oliver J. Schulwitz, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Mar. 3, 1958, Ser. No. 718,486 6 Claims. (Cl. 162157) Partially acetylated cellulose fibers Beat in water forming slurry Papering out Paper Wet with non-acid low boiling liquid Paper pressed and dried Paper is ordinarily prepared from cellulose fibers such as wood pulp by applying a slurry of the fibers to the wire of a paper making machine from which the thusformed paper is led eventually to calender rolls to impart a smooth, even finish thereto. To improve the characteristics of such paper, sizing agents may be applied either to the paper or to the fibers prior to their formation into paper. Partially acetylated cellulose paper often exhibits considerable resistance to sizing operations.

One object of my invention is to provide a partially acetylated paper useful for photographic purposes, the wet strength and fold values of which are comparable with, or better than, those of papers normally used as a photographic paper base. Another object of my invention is to provide a paper which is highly resistant to theeffects of moisture with respect to dimensional stability. Water resistant paper is of value particularly when employed photographically to withstand change or distortion in the picture or image which is supported by the paper. Other objects of my invention will appear herein.

I have found that paper having highly desirable properties may be prepared from ordinary grades of pulp if the fibers of that pulp are initially subjected to a partial acetylation so as to impart an acetyl content of 15-30% thereto and subsequently formed into a paper sheet which is then treated with a suitable organic liquid. By treatment in this fashion, sheets are obtained having good wet strength properties and fold characteristics and greatly improved Mullen and other valuable properties.

The cellulose fiber employed is kraft pulp, sulfite pulp, cotton fiber, cotton linters, linen fibers, bagasse etc., which has been partially acetylated such as by some method known in the art. Procedures of this type are described for instance in British Patent No. 367,312, or in an article entitled Effect of Acetylation on Water Binding Properties of Cellulose, by Lt. Aiken in Industrial and Engineering Chemistry, volume 35, pages 1206-1210, November 1943. After the cellulose fibers have been partially acetylated such as to an acetyl content of 15-30%, the fibers are formed into a sheet by any desirable paper making procedure such as in a hand sheet mold or on the Wire of a paper machine. The paper thus prepared after drying is treated with an organic liquid which acts to improve the strength characteristics of the paper.

Our invention also includes paper in which other fibers have been mixed with the partially acetylated fibers in its manufacture, the latter being at least half of the total fibers present. Some examples of other fibers which may be mixed with the partially acetylated fibers are those of refined wood pulp such as kraft pulp or high a-cellulose sulfite pulp, cotton fibers, nylon threads or fibers, copolymer fibers as described in application Serial No. 478,886 of Coover and Wooten, now Patent No. 2,831,- 826, often referred to as Verel fibers, linen fibers, fibers from reclaimed rags, etc.

The liquids which are useful for treating partially acetylated paper in accordance with my invention are those which either are actual solvents or are swelling agents for cellulose triacetate and which are readily removable from the paper sheet. A solvent which has been found to be exceptionally useful in this connection is methylene chloride-methanol, 95 :5 by weight. However, mixtures of methylene chloride and methanol in other ratios such as 98:2, :10 and the like, are useful. Other organic liquids which have been founld to be useful in this connection are methylene chloride per se, acetone, chloroform, methyl acetate and the like. Organic liquids which are higher boiling than those mentioned, but which are also useful for treating the paper are tetrachloroethane, ethylene chloride, methylethyl ketone, and acetic and formic acids. The fatty acids are limited in their application in that they tend to corrode many materials commonly use in paper machinery. In the case of the higher boiling liquids, it may sometimes be desirable after treatment to remove the liquid from the paper by means of a reduced pressure whereas with the lower boiling organic liquids removal may be accomplished at substantially atmospheric pressure.

The organic liquid may be applied to the sheet in any manner which will provide sufiicient of the liquid to accomplish the desired result and which will assure a uniform distribution of the liquid so that all parts of the paper receive the treatment. Methods of application of the liquid to the paper are spraying, applicator rolls, contact with felts or blotting papers containing solvents to be transferred to the paper, condensation of vapor on the sheet, and the like. The application to the paper of approximately two parts of organic liquid per part of paper has been found to be very satisfactory, but wide variations from these proportions occur in various applications.

Ordinarily, an amount of liquid within the range of 1 to 2 /2 parts of liquid per part of paper gives a useful effect thereon. Although the treatment of the sheet with organic liquid is usually accompanied by some sacrifice in tear, nevertheless, its resistance to expansion upon immersion in water is retained or improved. If the organic liquid is properly selected the treatment of the paper sheet always results in an improvement in the wet strength and Mullen value of the partially acetylated paper.

The following examples illustrate the advantage of treating paper sheets made from partially acetylated cellulose fibers with a liquid in accordance with my invention. Wet strength in all the examples was measured with a Minden paper tester.

3 Example 1 Shreds of bleached kraft pulp of ordinary grade had been acetylated by fibrous esterification to an apparent acetyl content of 21.6%. This acetylated fibrous material was suspended in water, beaten in 2. Valley beater to 30 seconds slowness, and was formed into handsheets weighing 20 pounds per 1,000 square feet on a British handsheet mold, the paper sheets thus obtained being pressed and dried. A sheet of this paper laid on a supporting surface of chrome steel, was dipped in a bath of methylene chloride following which the sheet was pressed between blotters in a hand wringer, and then subjected to pressing in a hydraulic press at a temperature of 60 C. until all the liquid had been removed from the sheet. The properties of the paper sheets with and without the treatment with the methylene chloride are shown by the data of the following table:

Shreds of bleached kraft pulp of ordinary grade which had been acetylated to an apparent acetyl content of 26.3% were slurried in water and the mass was beaten to a slowness of 30 seconds. The slurry was then sheeted on a British handsheet mold to give paper sheets weighing 21 pounds per 1,000 square feet, which sheets were dried. A dry sheet of the paper was laid on a stainless steel plate which was covered with enough methylene chloride to saturate the paper. The sheet was removed, covered with a blotter and pressed in a hydraulic press at 60 C. until the methylene chloride had all been removed therefrom. The properties of these sheets with and without the treatment with methylene chloride are shown below:

2-minute Wet Tear 1 Mullen 1 Folds 2 Wet ex- Strength pension Untreated 3. 78 0. 87 6 60 CH Ol -treated 2. 91 2. 71 443 55 13 1 Points per lb. per 1,000 sq. ft. 1 MIT double folds.

Example 3 Shreds of bleached kraft pulp of ordinary grade which had been acetylated in the fibrous state to a content of 27.1% apparent acetyl were placed in water and were beaten to form a slurry. Sheets were prepared therefrom and one of the sheets was treated with methylene chloride in accordance with the method described in Example 1. Tests upon this product in comparison with handsheets which had not been solvent treated, both with and without treatment in a hydraulic press are as follows:

Bleached fibrous kraft wood pulp which had been acetylated to an apparent acetyl content ot'22;4% was mixed with water and beaten in a Valley beater to form a slurry having 33 seconds slowness. Sheets having a weight of 20 pounds per 1,000 square feet were prepared on a British handsheet mold. Sheets thus prepared were laid upon a chrome steel support surface wet with a 95:5 methylene chloridezmethanol mixture by weight. The sheets were then placed between blotters and dried with and without the application of pressure during the solvent removal.

Sheets were also prepared in like manner except that the solvent treatment was not given to the paper sheets prepared. These sheets were subjected to a series of tests and the results obtained were as follows:

Fibers of bleached kraft pulp which had been acetylated to an apparent acetyl content of 19.6% were formed into a slurry by beating in a Valley beater with water to a slowness of 30 seconds. This partially acetylated pulp was formed into handsheets of 20 pounds per 1,000 square feet basis weight. These sheets were laid on a supporting surface of a chrome steel plate and covered with enough 95 :5 methylene-chloridezmethanol to saturate the sheets. The solvent laden sheets were placed between blotters and pressed at 60 C. at 55 pounds per square inch until the solvent had evaporated. Both the solvent-treated handsheets and untreated comparisons were run through calender rolls before testing. Improvement in the properties both dry and wet of the sheets as compared with those which had not been solvent treated A mixture of partially acetylated refined wood pulp and 10% unacetylated refined wood pulp was formed into a slurry by beating with water in a Valley beater to the desired slowness. This pulp mixture was then formed into paper sheets having a basis weight of 20 pounds per 1,000 square feet. The sheets were then saturated with a mixture of methylene chloride and 5% methanol, in which solvent mixture 2% of cellulose acetate butyrate had been dissolved. The solvent-laden sheets were placed between blotters and pressed at 60 C. at 55 pounds per square inch until the solvent had been evaporated. Like sheets were prepared except that the paper was composed of partially acetylated refined wood pulp, which sheets were also treated with methylene chloride-methanol having cellulose acetate:

butyrate dissolved therein as described, following which the sheets were dried by evaporation of the solvent. The

sheets prepared both from 100% acetylated fibers and from the mixture of acetylated and unacetylated fibers were tested with the following results:

1. A method of preparing paper characterized by good wet strength, flexibility and Mullen value which comprises preparing sheet paper from an aqueous slurry formed by beating partially acetylated cellulose fiber having an acetyl content of 1530% in water, wetting the paper thus formed with a low boiling nonacid organic liquid which exerts, at least, a swelling action on cellulose triacetate followed by pressing the so-treated paper and removing the organic liquid therefrom.

2. A method of preparing paper characterized by good wet strength, flexibility and Mullen value which comprises preparing sheet paper from an aqueous slurry formed by beating partially acetylated cellulose fiber having an acetyl content of 1530% in water, wetting the paper thus formed with methylene chloride followed by pressing the so treated paper and removing the methylene chloride therefrom.

3. A method of preparing paper characterized by good wet strength, flexibility and Mullen value which comprises preparing sheet paper from an aqueous slurry formed by beating partially acetylated cellulose fibers having an acetyl content of 15-30% in water, wetting the paper thus formed with methylene chloride-alcohol followed by pressing the so treated paper and removing the methylene chloride-alcohol therefrom.

4. A method of preparing paper characterized by good wet strength, flexibility and Mullen value which comprises preparing sheet paper from an aqueous slurry formed by beating partially acetylated kraft pulp having an acetyl content of 1530% in water, wetting the paper thus formed with a low boiling nonacid organic liquid which exerts, at least, a swelling action on cellulose tnacetate followed by pressing the so treated paper and removing the organic liquid therefrom.

5. A method of preparing paper characterized by good wet strength, flexibility and Mullen value which comprises preparing sheet paper from an aqueous slurry formed by beating partially acetylated krafit pulp having an acetyl content of .1530% in water, wetting the paper thus formed with methylene chloride-alcohol followed by pressing the so treated paper and removing the methylene chloride-alcohol therefrom.

6. A method of preparing paper characterized by good wet strength, flexibility and Mullen value which comprises preparing sheet paper from an aqueous slurry formed by beating partially acetylated cellulose fiber having an acetyl content of 1530% in water, wetting the paper thus formed with methylene chloride-alcohol containing a small proportion of cellulose acetate butyrate dissolved therein followed by pressing the so treated paper and removing the methylene chloride-alcohol therefrom.

References Cited in the file of this patent UNITED STATES PATENTS 1,963,974 Ellis June 26, 1934 2,054,284 Forstmann Sept. 15, 1936 2,208,652 Whitehead July 23, 1940 2,282,415 Hadley et a1. May 12, 1942 2,357,962 Leemann Sept. 12, 1944 2,365,297 Schweizer Dec. 19, 1944 2,930,106 Wrotnowski Mar. 29, 1960 

2. A METHOD OF PREPARING CHARACTERIZED BY GOOD WET STRENGTH, FLEXIBLITY AND MULLEN VALUES WHICH COMPRISES PREPARING SHEET FROM AN AQUEOUS SLURRY FORMES BY BEATING PARTIALLY ACETYLATED CELLULOSE FIBER HAVING AN ACETYL CONTENT OF 15-30% IN WATER, WETTING THE PAPER THUS FORMED WITH METHYLENE CHLORIDE FOLLOWED BY PRESSING THE SO TREATED PAPER AND REMOVING THE METHYLAENE CHLORIDE THEREFROM. 